diff --git a/include/gsl/span b/include/gsl/span
index b356ee9..54f5f6b 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -556,7 +556,11 @@ private:
         // wrap around to a value always greater than size when casted.
 
         // check if we have enough space to wrap around
+#if defined(__cpp_if_constexpr)
+        if constexpr (sizeof(index_type) <= sizeof(size_t))
+#else
         if (sizeof(index_type) <= sizeof(size_t))
+#endif
         {
             return narrow_cast<size_t>(idx) < narrow_cast<size_t>(size);
         }